1. Technical Field of the Invention
The present invention relates to a method for manufacturing a stator used, for example, for rotary electric machines, and in particular to a method for manufacturing a stator for radially-gapped inner-rotor type rotary electric machines.
2. Related Art
Recently, it is essential for the automotive industry to give serious consideration to the environment and power saving. Under such circumstances, EVs (electric vehicles) and HVs (hybrid vehicles) have been researched and put to practical use.
In vehicles, such as EVs and HVs, it is important to reduce vehicle weight that has a strong positive correlation with fuel consumption. In this regard, there has been a strong demand for enhancing output per unit weight (kW/kgw) of a high-current motor (e.g., drive motor) loaded on such a vehicle. It has been known that, in a stator incorporated in such a motor, the increase in a space factor of slots, i.e. the cross-sectional area of the coil conductor per unit cross-sectional area of slots, in the stator, has a strong positive correlation with the output per unit weight of the motor (kW/kgw).
One known method for winding a coil about a stator involves use of a coil winder. In the case of a conventional winding method, i.e. in the case of winding a fine round wire about one slot, using such a coil winder, it is necessary to ensure a space in the slot, to which the coil winder is inserted. In addition, the cross section of the coil conductor has a circular shape. For these reasons, the space factor of slots (hereinafter referred to “slot space factor”) of a stator about which a coil is wound has been as small as about 40%.
It should be appreciated that, throughout the specification, when a term “cross section” or “cross-sectional area” is used for a winding or a wire, the term refers to a cross section or a cross-sectional area perpendicular to the longitudinal direction of the wire.
If a flat wire having a large cross-sectional area can be used as a stator coil, the slot space factor may be remarkably improved. However, such a large-size flat wire cannot be wound about a stator with the use of a coil winder, unlike the case of winding a normal fine round wire. On the contrary, bending, per se, has been difficult in winding such a large-size flat wire. Therefore, when a stator is manufactured using a large-size flat conductor wire, the method may preferably include a step of bending the large-size flat conductor wire for preparation of a stator coil in advance, followed by a step of inserting portions of the conductor wire of the coil to be accommodated in slots (herein after referred to as “slot-accommodated portions”) into the respective slots of the stator core, for completion of a stator.
Stator coils are known to have concentrated winding or distributed winding. Comparing with the latter, the former is known to have an advantage, for example, of reducing torque ripple. A stator coil having a distributed winding of a large-size flat conductor wire will have a shape of a cage (hereinafter referred to a “cage stator coil” or a “cage coil”). The outer diameter of such a cage stator coil will be approximately equal to that of the bottom surface of the slots of the stator core. For this reason, the stator coil cannot be easily inserted into a cylindrical stator core. Specifically, a cage stator coil prepared using a large-size flat square conductor wire of large cross-sectional area will have a very large rigidity. Therefore, it will be difficult to dispose the slot-accommodated portions, as they are, so as to be located radially inside the stator core having a relatively small inner diameter.
An approach known for avoiding the difficulty mentioned above in mounting a cage stator coil in the slots of a stator core, may be to use a divided core structure. Specifically, in this approach, a stator is divided into a plurality of segment cores (also referred to as “divided cores”). Then, the slot-accommodated portions of the cage stator coil are inserted into the slots of each divided core. After that, the divided cores are mechanically connected to complete a stator. Owing to the combination of such a divided-core type stator with a cage stator coil, an insulation-coated flat conductor wire having a large cross-sectional area can be used as a coil conductor to realize a radially-gaped inner-rotor type rotary electric machine having a good slot space factor. However, use of such divided cores may raise a lot of problems, such as increase of magnetic resistance. Also, reducing the number of divisions of a core means that difficulty may be caused in the insertion of a stator coil.
In this regard, Japanese Patent No. 3982446 suggests a method for assembling a stator coil. According to the assembling method disclosed in this reference, a cage stator coil whose outer diameter is smaller than the inner diameter of a stator core is inserted into the stator core. After the insertion, the diameter of the stator coil is expanded, whereby the slot-accommodated portions are thrust into the respective open slots of the stator core. Hereinafter, this method is simply referred to as an “open-slot thrust method”.
However, with the open-slot thrust method mentioned above, two coil ends at both ends of the cage stator coil are required to be biased axially inward while being radially expanded. Further, use of a large-size flat conductor wire to form coil end portions configuring the coil ends, may necessitate plastic deformation of the coil ends with an extraordinarily large force.
This means that large bending forces are concentrated on each of the coil ends, leading to a concern that damage may be caused to the insulation coating at the surface of each coil end which is brought into contact with a bending/biasing end portion of a bending device. Additionally, since the elasticity of the coil ends is large, there has also been a great concern that the coil ends that have been bent radially outward will spring back for restoration to permit the slot-accommodated portions of the stator coil to come out from the slot openings to the inside.